34  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
same  time,  in  the  neighborhood  of  known  mineralization  further 
search  along  the  strike  is  the  most  logical  procedure. 
The  main-lode  system,.- — The  most  prominent  feature  of  the  Juneau 
gold  belt  is  a  system  of  stringer  leads,  which  resembles  the  mother 
lode  of  California,  It  occupies  a  band  of  variable  width  in  black 
slates  lying  northeast  of,  that  is,  stratigraphically  above,  a  thick  group 
of  greenstone  beds,  which  includes  the  highest  of  the  basaltic  flows  of 
the  slate-greenstone  series.  This  group  of  slates  and  the  underlying 
greenstones  have  been  traced  throughout  the  entire  length  of  the 
Juneau  belt  from  Windham  Bay  almost  to  Berners  Ba}%  where  they 
are  cut  out  b}^  the  transverse  contact  of  the  main  diorite  of  the  Coast 
Range,  and  at  many  points  throughout  this  distance  a#great  deal  of 
veining  and  mineralization  is  found. 
The  greenstones  are  locally  schistose,  and  where  they  have  this 
structure  they  are  sometimes  impregnated  along  surfaces  of  particu- 
larly intense  sheeting  with  sulphides  locally  accompanied  by  gold.  In 
such  cases  quartz  is  seldom  present.  Elsewhere  when  the  rocks  are 
unaffected  by  secondary  structure  they  contain  stringers,  or  even 
fairly  well-defined  veins  of  quartz,  which  sometimes  carry  the  same 
values  as  the  veins  in  the  main  lode  system. 
The  slates  in  which  the  quartz  veins  of  the  main  lode  are  found  bear 
a  general  similarity  to  those  occuring  throughout  the  stratigraphic 
band  of  which  they  form  a  part.  They  are  highly  metamorphosed, 
and  having  been  originally  carbonaceous  now  contain  large  amounts 
of  graphite,  which,  with  other  secondary  platy  minerals,  is  arranged 
in  conformity  with  the  slaty  structure  of  the  rock.  As  a  whole  the 
slates  are  but  little  plicated,  and  their  secondary  cleavage  follows  the 
original  bedding,which  in  turn  is  parallel  with  the  greenstone  contact. 
Locally,  as  in  the  Gold  Creek  mines,  where  dikes  are  intruded  into 
them,  there  is  some  intricate  folding,  and  when  this  feature  is  present 
the  secondary  structures  do  not  conform  to  the  original  bedding,  but 
keep  their  normal  attitude  parallel  to  the  average  position  of  the 
stratigraphic  planes. 
The  quartz  veins  which  compose  the  lode  are  not  continuous  for  long 
distances,  but  occur  as  independent  lenses,  or  in  series  of  interrupted 
overlapping  veins  closely  following  the  structure  of  the  slates  within 
a  zone  which  may  often  attain  a  width  of  100  feet,  or  again  in  the  form 
of  stringers  filling  gashes  slightly  oblique  to  the  main  structure,  but 
ranged  in  sets  along  some  particular  horizon.  Often  the  large  lenses 
or  stringers  are  joined  by  networks  of  minor  veinlets  which  follow 
secondary  joints  arranged  with  more  or  less  regularity. 
MINES   OF    THE    DISTRICT. 
Gold  Creek  mines. — The  lode  system,  which  has  been  described  in 
general  terms,  is  most  strongly  developed  in  the  vicinity  of  Juneau, 
